This invention relates to metal joining processses and more particularly to elimination of gaps formed in inaccessible areas of welded joints.
Housings for certain valves, in particular valves for use in large metal water pipe systems for underground applications, have a structure which includes an inner tubular pipe portion and an outer generally tubular portion coaxial with the inner portion and overlapped over a set distance adjacent to an end of the inner pipe, the outer tubular portion being spaced apart radially a substantial distance from the inner pipe at the overlapped inner pipe end and tapered inwardly so as to come into contact with and to be joined to the inner pipe at the opposite end of the overlapped portion. The inwardly tapered end of the outer pipe may be joined to the inner pipe surface by welding performed from the outside surface of the outer pipe, this side of the joint being readily accessible to necessary welding equipment. While a single welded joint applied in this manner may meet initial requirements relating to strength and water-tight capability, a long-term problem is presented owing to the adverse effect of such a joint not obtaining complete coverage of inside surfaces with a protective coating. Use of double welding wherein a second welding step performed along the inside surface of the outer pipe would not be feasible owing to the cramped available space. For example, access at a forty-five degree angle, as required to lay down a fillet for a back-side welded joint, is unavailable in this structure.
A protective coating applied to inside surfaces of the valve housing is required to prevent corrosion as would occur in carbon steel pipes exposed to water. Coatings for this purpose typically comprise a baked powder coating applied by air bath. When applied to joint surfaces at a backside location at a single-welded joint, coatings of this type may fail to provide complete coverage of surfaces inside of minute gaps along the backside of the joint interface. This lack of complete penetration of the coating is believed to result from the coating being electrostatically attracted to one side or the other before it gets to the corner. In effect the exposed surfaces within gaps create a latent defect, shortening the service life of the valve housing.
The present invention is directed to a process for fabricating a valve housing wherein an inner tubular steel member is joined to an outer steel member, the outer member disposed generally coaxial with the inner member and having an end portion overlapping the inner member, and the end portion being tapered inward so as you come into contact with an inner end of the overlapped area. The two members are joined by being placed in aligned position with a side edge surface of the outer member in contact with a mating surface of the inner member, welding the members together by means of a single weld applied from the direction of the outside surface of the outer member and treating the joint area on the side opposite to the single weld to remove gaps at the juncture of the inner side of the outer member and the inner member. After removal of gaps, a protective coating is then applied to surfaces of the housing to prevent corrosion.
Treatment of the juncture area opposite from the single welded side to remove gaps may be carried out by a brazing step performed after welding of the accessible side or by obtaining a high precision fit between the joining members prior to welding by means of shrink-fit or mechanical press-fit techniques.
xe2x80x9cBrazingxe2x80x9d as used herein is intended to refer to a group of welding processes wherein the filler metal is a nonferrous metal or alloy whose melting point is higher than 1000 degrees Fahrenheit, but lower than that of the metals or alloys to be joined.
Further details regarding specific filler metals and process conditions are given below. Terminology used in describing applicable process is consistent with information disclosed in Van Nostrand""s Scientific Encyclopedia, Fourth Edition at page 236, and attributed to the American Welding Society.
Brazing may be carried out by placement of a suitable alloy having a melting point below the melting point of the steel members in position around the periphery of the single-welded joint and in position to move into gaps at the backside of the welded juncture by capillary action and/or gravity and diffusion. Heating the assembly to a temperature such as 1800 to 1900 degrees Fahrenheit in an induction furnace may be used for this purpose.
A high-precision gap-free backside juncture may also be obtained by use of a shrink-fitting procedure wherein the joint areas of the members are machined to a tolerance such as 0.010 to 0.020 inch and the outer member is caused to expand by being heated to a high temperature and, while expanded is moved in place over the inner member, which is not heated, but may instead be cooled. Upon allowing the outer member to cool, an effective interference fit is obtained.
Removal or prevention of gaps at joint areas in a welded valve housing in accordance with this invention enables complete coverage of interior housing surfaces with protective coatings as are required for prevention of corrosion. Long term service life of the housing is thus enhanced.
It is therefore an object of this invention to provide a process wherein gaps in inaccessible backside areas of single welded joints are avoided.
Another object is to provide a process for fabricating valve housings from overlapped tubular steel members wherein inaccessible backside areas of welded joints are treated to remove minute gaps.
Another object is to provide a process for fabricating valves for water pipe service wherein complete coverage of housing surfaces with a protective coating is enabled.